Memory device comprising an array portion and a logic portion

ABSTRACT

In an embodiment of the present invention, a method comprises patterning a first plurality of semiconductor structures in an array portion of a semiconductor substrate using a first photolithographic mask. The method further comprises patterning a second plurality of semiconductor structures over a logic portion of a semiconductor substrate using a second photolithographic mask. The method further comprises patterning a sacrificial layer over the first plurality of semiconductor structures using the second photolithographic mask. The sacrificial layer is patterned simultaneously with the second plurality of semiconductor structures.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/940,948, now U.S. Pat. No. 8,207,583, filed 5 Nov. 2010, which is adivisional of U.S. patent application Ser. No. 11/367,020, now U.S. Pat.No. 7,842,558, filed 2 Mar. 2006.

This application is also related to U.S. patent application Ser. No.10/933,062, filed 1 Sep. 2004, now U.S. Pat. No. 7,442,976, U.S. patentapplication Ser. No. 10/934,778, filed 2 Sep. 2004, now U.S. Pat. No.7,115,525, U.S. patent application Ser. No. 10/855,429, filed 26 May2004, now U.S. Pat. No. 7,098,105, U.S. patent application Ser. No.11/201,824, filed 10 Aug. 2005, now U.S. Pat. No. 7,391,070, and U.S.patent application Ser. No. 11/366,212, filed concurrently with theparent application, now U.S. Pat. No. 7,476,933. The entire disclosureof each of these related applications is hereby incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates generally to methods for formingsemiconductor structures, and relates more specifically to improvedmethods for forming vertical transistor devices.

BACKGROUND OF THE INVENTION

One way that integrated circuit designers make faster and smallerintegrated circuits is by reducing the separation distance between theindividual elements that comprise the integrated circuit. This processof increasing the density of circuit elements across a substrate istypically referred to as increasing the level of device integration. Inthe process of designing integrated circuits with higher levels ofintegration, improved device constructions and fabrication methods havebeen developed.

An example of a common integrated circuit element is a transistor.Transistors are used in many different types of integrated circuits,including memory devices and processors. A typical transistor comprisesa source, a drain, and a gate formed at the substrate surface. Recently,vertical transistor constructions that consume less substrate “realestate”, and thus that facilitate increasing the level of deviceintegration, have been developed. Examples of vertical transistorconstructions are disclosed in U.S. patent application Ser. No.10/933,062 (filed 1 Sep. 2004; Attorney Docket MICRON.299A; MicronDocket 2004-0398.00/US), the entire disclosure of which is herebyincorporated by reference herein. While these improved transistorconstructions are smaller and are packed more densely, they also ofteninvolve fabrication processes that are significantly more complex,therefore increasing fabrication time and expense. Fabricationcomplexity is increased even further when high density verticaltransistors are formed in an array on the same substrate as logiccircuitry that is positioned adjacent to the transistor array. Inparticular, conventional fabrication techniques use separate masks toindependently define features in the device array region and in thedevice periphery region, since different process steps and materials areused to define the devices of these two regions.

Conventional semiconductor-based electronic storage devices, such asdynamic random access memory (“DRAM”) devices, include large numbers oftransistor and capacitor elements that are grouped into memory cells.The memory cells that comprise a DRAM device are arranged into largermemory arrays that often comprise thousands, if not millions, ofindividual memory cells. Therefore, there is a continuing effort toreduce the complexity of the processes used to form densely-packedintegrated circuit elements such as vertical transistor constructions.

BRIEF SUMMARY OF THE INVENTION

According to one embodiment of the present invention, a method offorming an array of memory devices comprises forming a plurality of deeptrenches and a plurality of shallow trenches in a first region of asubstrate. At least one of the shallow trenches is positioned betweentwo deep trenches. The plurality of shallow trenches and the pluralityof deep trenches are parallel to each other. The method furthercomprises depositing a layer of conductive material over the firstregion and a second region of the substrate. The method furthercomprises etching the layer of conductive material to define a pluralityof lines separated by a plurality of gaps over the first region of thesubstrate, and a plurality of active device elements over the secondregion of the substrate. The method further comprises masking the secondregion of the substrate. The method further comprises removing theplurality of lines from the first region of the substrate, therebycreating a plurality of exposed areas from which the plurality of lineswere removed. The method further comprises etching a plurality ofelongate trenches in the plurality of exposed areas while the secondregion of the substrate is masked.

According to another embodiment of the present invention, an apparatuscomprises a semiconductor substrate having an array portion and a logicportion. The apparatus further comprises at least one U-shapedsemiconductor structure formed in the substrate array portion. Thesemiconductor structure comprises a first source/drain region positionedatop a first pillar, a second source/drain region positioned atop asecond pillar, and a U-shaped channel connecting the first and secondsource/drain regions. The U-shaped channel is contiguous with thesemiconductor substrate. The method further comprises at least onetransistor device formed over the substrate logic portion, thetransistor device including a gate dielectric layer and a gate material.The gate dielectric layer is elevated with respect to the first andsecond source/drain regions.

According to another embodiment of the present invention, a memorydevice comprises a substrate having an array portion and a logicportion. The memory device further comprises a plurality of U-shapedsemiconductor structures that are formed in the array portion of thesubstrate. The U-shaped semiconductor structures are defined by apattern of alternating deep and shallow trenches that are crossed by apattern of intermediate-depth trenches. The memory device furthercomprises a plurality of transistor devices formed over the logicportion of the substrate. The transistor devices include a gate oxidelayer, an uncapped gate layer, and a sidewall spacer structure.

According to another embodiment of the present invention, a methodcomprises patterning a plurality of shallow trenches and a plurality ofdeep trenches in a substrate array region. The method further comprisespatterning a plurality of intermediate-depth trenches in the substratearray region. The intermediate-depth trenches cross the shallow and deeptrenches. The intermediate-depth, shallow and deep trenches define aplurality of U-shaped transistor structures in the substrate arrayregion. The plurality of intermediate-depth trenches are defined by aphotolithography mask. The method further comprises patterning aplurality of planar transistor structures in a substrate logic region.The plurality of planar transistor structures are defined by thephotolithography mask.

According to another embodiment of the present invention, a methodcomprises patterning a first plurality of semiconductor structures in anarray portion of a semiconductor substrate using a firstphotolithographic mask. The method further comprises patterning a secondplurality of semiconductor structures over a logic portion of asemiconductor substrate using a second photolithographic mask. Themethod further comprises patterning a sacrificial layer over the firstplurality of semiconductor structures using the second photolithographicmask. The sacrificial layer is patterned simultaneously with the secondplurality of semiconductor structures.

According to another embodiment of the present invention, a methodcomprises providing a semiconductor substrate having a first region anda second region. The method further comprises depositing a conductivelayer over the substrate first and second regions. The method furthercomprises patterning the conductive layer deposited over the substratefirst and second regions. The method further comprises using thepatterned conductive layer to form a planar transistor structure overthe substrate second region. The method further comprises using thepatterned conductive layer in a masking process in the substrate firstregion.

According to another embodiment of the present invention, apartially-formed integrated circuit comprises a first plurality offeatures comprising a first material and formed over a first portion ofa substrate. The first plurality of features are separated from eachother by a first spacing. The partially-formed integrated circuitfurther comprises a second plurality of features comprising a secondmaterial and formed over a second portion of the substrate. The firstplurality of features and the second plurality of features are formedsimultaneously. The first material is the same as the second material.The partially-formed integrated circuit further comprises a gap fillstructure positioned between and contacting a selected two of the firstplurality of features. The partially-formed integrated circuit furthercomprises a plurality of sidewall spacers positioned adjacent the secondplurality of features. Adjacent sidewall spacers are separated from eachother by a separation region. The plurality of sidewall spacers and thegap fill structure comprise the same material.

According to another embodiment of the present invention, a memorydevice comprises a substrate having an array portion and a logicportion. The memory device further comprises a plurality ofsemiconductor structures that are recessed in the array portion of thesubstrate. The memory device further comprises a plurality of transistordevices formed over the logic portion of the substrate. The transistordevices include a gate oxide layer, an uncapped gate layer, and asidewall spacer structure. The transistor devices are formed in a layerthat is below the plurality of semiconductor structures.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the transistor constructions disclosed hereinare illustrated in the accompanying drawings, which are for illustrativepurposes only. The drawings comprise the following figures, in whichlike numerals indicate like parts.

FIG. 1 illustrates a perspective view of a partially-formedsemiconductor device usable to form an array of transistors.

FIG. 2 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 1, after the formation ofadditional semiconductor processing layers.

FIG. 3 illustrates a partial top plan view of an exemplary embodiment ofa photo mask to be applied to the partially-formed semiconductor deviceof FIG. 1.

FIG. 4 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 2 after the photo mask ofFIG. 3 has been applied and transferred to pattern the hard mask layer.

FIG. 5 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 4 after blanket depositinga layer of spacer material thereover.

FIG. 6 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 5 after performing adirectional etch of the spacer material.

FIG. 7 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 6 after etching aplurality of deep trenches into the substrate.

FIG. 8 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 7 after filling the deeptrenches with a dielectric material and providing the device with asubstantially planar surface.

FIG. 9 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 8 after patterning a hardmask layer thereover.

FIG. 10 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 9 after forming aplurality of spacers on the vertical sides of the patterned hard masklayer.

FIG. 11 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 10 after etching aplurality of shallow trenches into the substrate.

FIG. 12 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 11 after filling theshallow trenches with a dielectric material and providing the devicewith a substantially planar surface.

FIG. 13 illustrates a top-down view in the xy plane of thepartially-formed semiconductor device of FIG. 12.

FIG. 14 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 12 after removing residualmasking layers.

FIG. 15 illustrates a cross-sectional view in the xz plane of thepartially-formed semiconductor device of FIG. 14, taken along line15-15, after depositing gate stack layers thereover.

FIG. 16 illustrates a cross-sectional view in the xz plane of thepartially-formed semiconductor device of FIG. 15 after patterning activedevices in the periphery region and lines in the array region.

FIG. 17 illustrates a cross-sectional view in the xz plane of thepartially-formed semiconductor device of FIG. 16 after forming spacermaterial around the periphery region active devices and between thearray region lines.

FIG. 18 illustrates a cross-sectional view in the xz plane of thepartially-formed semiconductor device of FIG. 17 after masking thedevice periphery region and etching gate stack layers from the unmaskedarray portions of the device.

FIG. 19 illustrates a cross-sectional view in the xz plane of thepartially-formed semiconductor device of FIG. 18 after shrinking theremaining spacer material using a isotropic etch.

FIG. 20 illustrates a cross-sectional view in the xz plane of thepartially-formed semiconductor device of FIG. 19 after etching a patternof intermediate trenches into the structure illustrated in FIG. 14.

FIG. 21 illustrates a cross-sectional view in the xz plane of thepartially-formed semiconductor device of FIG. 20 after removingremaining spacer material from the array region, lining the intermediatetrenches with a dielectric, and forming sidewall spacers of gatematerial in the intermediate trenches.

FIG. 22 illustrates a perspective view of a portion of thepartially-formed semiconductor device of FIG. 21.

FIG. 23 illustrates a perspective view of one transistor comprising thepartially-formed semiconductor device of FIG. 22, including an overlyingcapacitor and bit line.

FIG. 24 illustrates a cross-sectional view in the xz plane of thepartially formed semiconductor device in an embodiment wherein aself-aligned silicidation process is used to create a silicide region onpolycrystalline gate stacks.

FIG. 25 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 8 after etching thenitride layer in the array region.

FIG. 26 illustrates a cross-sectional view in the yz plane of thepartially-formed semiconductor device of FIG. 25 after forming nitridespacers around the protruding spin-on-dielectric material.

FIG. 27 is a schematic plan view of a memory device that illustrates theposition of a memory cell with respect to an array of bit lines and wordlines.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein are improved fabrication techniques for verticaltransistor constructions. As disclosed above, vertical transistorconstructions advantageously enable increased levels of deviceintegration. The fabrication techniques disclosed herein advantageouslyuse (a) fewer masking processes as compared to conventional fabricationtechniques, and/or (b) masking processes that are easier to align. Forexample, certain of the embodiments disclosed herein advantageouslyenable the forming of active devices in the periphery region andpatterning features (for example, intermediate trenches separating rowsof transistors) in the array region with a single mask. Additionally,certain embodiments of the vertical transistors disclosed herein have aU-shaped configuration, wherein the channel connecting the source anddrain regions is directly connected to the underlying substrate. Thisadvantageously reduces or eliminates the floating body effect that iscommon in conventional vertical pillar transistors.

The U-shaped vertical transistor configurations disclosed herein provideseveral advantages over conventional planar transistors. In addition toconsuming less substrate “real estate”, certain of the U-shaped verticaltransistor configurations disclosed herein form continuous rows andcolumns during fabrication, thereby enhancing the structural stabilityof the device. Certain embodiments of the fabrication techniquesdisclosed herein also advantageously allow use of a simplified reticleset to perform the masking processes employed to fabricate the memoryarray. Specifically, one embodiment of the reticle set used to fabricatesuch an array contains parallel lines and spaces, thereby facilitatingprinting and alignment of the masking processes.

The techniques disclosed herein are usable to form transistor structureswith a wide variety of different dimensions. In certain embodiments,pitch doubling techniques are used to form relatively smaller devices inan array region, and conventional photolithography techniques are usedto form relatively larger devices in a periphery region. For example, inone embodiment structures having a feature size between ½F and ¾F areformed in the array region, while structures having a feature size of For larger are formed in the periphery region, wherein F is the minimumresolvable feature size obtainable using a given photolithographytechnique. Additional information regarding pitch doubling techniquesare provided in U.S. patent application Ser. No. 10/934,778 (filed 2Sep. 2004; Attorney Docket MICRON.294A; Micron Docket 2003-1446.00/US),the entire disclosure of which is hereby incorporated by referenceherein.

FIG. 1 is a perspective view of a partially formed semiconductor device100 in which a transistor array is to be formed. In one embodiment, thedevice 100 comprises a memory array, such as an array of DRAM cells,although in other embodiments the device 100 comprises an array of othertypes of memory cells, such as static memory cells, dynamic memorycells, extended data out (“EDO”) memory cells, EDO DRAM, electricallyerasable programmable read only memory (“EEPROM”) cells, synchronousdynamic random access memory (“SDRAM”) cells, double data rate (“DDR”)SDRAM cells, synchronous link dynamic random access memory (“SLDRAM”)cells, video dynamic random access memory (“VDRAM”) cells, RDRAM® cells,static random access memory (“SRAM”) cells, phase change or programmableconductor random access memory (“PCRAM”) cells, magnetic random accessmemory (“MRAM”) cells, and flash memory cells.

The device 100 includes a semiconductor substrate 110, which comprisesone or more of a wide variety of suitable semiconductor materials. Inmodified embodiments, the semiconductor substrate 110 includessemiconductor structures that have been fabricated thereon, such asdoped silicon platforms. While the illustrated semiconductor substrate110 comprises an intrinsically doped monocrystalline silicon wafer inthe illustrated embodiment, in other embodiments the semiconductorsubstrate 110 comprises other forms of semiconductor layers, whichoptionally include other active or operable portions of semiconductordevices.

Optionally, an epitaxial layer 104 is grown on the substrate 110. Theepitaxial layer 104 is a semiconductor layer (for example, comprisingsilicon) grown on the substrate 110 by an epitaxial growth process thatextends the crystal structure of the substrate 110. The epitaxial layer104 has a thickness that is preferably between about 2 μm and about 6μm, and more preferably between about 3 μm and about 5 μm. Inembodiments wherein the epitaxial layer 104 is grown on the substrate110 before the subsequent etching steps described herein, the epitaxiallayer 104 is considered part of the substrate 110.

In certain embodiments, the epitaxial layer 104 is heavily doped with aconductivity type that is opposite that of the substrate 110, therebyenabling the epitaxial layer 104 to serve as an active area fortransistors formed thereover, as will be better understood from thefinal structures disclosed herein. In one configuration, the dopedimplant regions include a lightly doped p⁻ region that is positionedunderneath a heavily doped p⁺ region.

FIG. 2 illustrates a cross-section in the yz plane of the device of FIG.1 after deposition of additional layers over the substrate 110. Asillustrated, the semiconductor device 100 further comprises an oxidelayer 210 formed over the substrate 110 and the optional epitaxial layer104. In an exemplary embodiment, the oxide layer 210 is selectivelyetchable with respect to the material comprising the substrate 110 andsilicon nitride. In one embodiment, the oxide layer 210 comprisessilicon dioxide and has a thickness that is preferably between about 100A and 500 A, and more preferably between about 200 A and about 300 A.For example, in one embodiment, the oxide layer 210 is a pad oxide layerhaving a thickness of approximately 200 A. The oxide layer 210 isdeposited using a suitable deposition process, such as chemical vapordeposition (“CVD”) or physical vapor deposition (“PVD”), or is grown byoxidation of the underlying substrate.

Still referring to FIG. 2, the semiconductor device 100 furthercomprises a layer, such as the illustrated nitride layer 211, formedover the oxide layer 210. In one embodiment, the nitride layer 211comprises silicon nitride and has a thickness that is preferably betweenabout 200 Å and 2000 Å, and more preferably between about 500 Å and 1000Å. The nitride layer 211 is deposited using a suitable depositionprocess, such as CVD or PVD.

The semiconductor device 100 further comprises a further hard mask layer212 that is formed over the nitride layer 211. In an exemplaryembodiment, the hard mask layer 212 comprises amorphous carbon. In otherembodiments, the hard mask layer 212 comprises transparent carbon,tetraethylorthosilicate (“TEOS”), polycrystalline silicon, Si₃N₄,SiO_(x)N_(y), SiC, or another suitable hard mask material. The hard masklayer 212 is deposited using a suitable deposition process, such as CVDor PVD. For purposes of clarity, the optional epitaxial layer 104 isomitted from subsequent illustrations.

FIG. 3 illustrates a portion of a photo mask 300 to be applied to thedevice 100 to pattern the underlying hard mask layer 212. The shadedportion of the photo mask 300 represents the area in which the hard masklayer 212 will be removed after applying photolithography and etchingtechniques, and the unshaded portion represents the area in which thehard mask layer 212 will remain. The photo mask 300 is a clear fieldmask that is configured to define a pattern of active area lines 304separated from each other by gaps 302 in an array region 308.Preferably, the lines 304 and the gaps 302 are approximately 1100 Å toapproximately 1300 Å wide. For example, in an exemplary embodiment thelines 304 and the gaps 302 are approximately 1200 A wide. The photo mask300 optionally includes a wider line 306 that is provided for opticalproximity correction. The gaps 302 are used as a contact area forshallow trench isolation.

FIG. 4 illustrates a cross-section in the yz plane of the device of FIG.2 after applying the photo mask 300, illustrated in FIG. 3, to patternthe hard mask layer 212. The photo mask 300 is applied and transferredto the hard mask layer 212, such that the lines 304 and gaps 302 extendparallel to the x axis. As illustrated in FIG. 4, the hard mask layer212 remains over areas of the substrate 110 where the photo mask 300forms lines 304, including the wider line 306, and is removed form areasof the substrate 110 where the photo mask 300 forms gaps 302. Asillustrated in FIG. 4, lines 304 and gaps 302 are located in an arrayregion 308 of the device, which is surrounded by a periphery region 310of the device.

In an exemplary embodiment, the hard mask layer 212 is patterned usingphotolithography and etching techniques. For example, in one embodimentphotoresist material is deposited as a blanket layer over the device100, and is exposed to radiation through a reticle. Following thisexposure, the photoresist material is developed to form the photo mask300, illustrated in FIG. 3, on the surface of the hard mask layer 212.The hard mask layer 212 is then etched through the photo mask 300 toexpose the nitride layer 211 of the device 100 in the gaps 302.

FIG. 5 illustrates a cross-section in the yz plane of the device of FIG.4 after blanket depositing a layer of spacer material 214 thereover. Inan exemplary embodiment, the spacer material 214 comprises an oxidematerial, such as silicon oxide having a thickness that is preferablybetween about 200 Å and about 500 Å, and more preferably between about300 Å and about 400 Å. In another embodiment, the spacer material 214fills approximately 1/20 to approximately ⅓ of the horizontal dimensionof the gaps 302. The spacer material 214 is deposited using a suitabledeposition process, such as CVD or PVD.

FIG. 6 illustrates a cross-section in the yz plane of the device of FIG.5 after preferentially etching the spacer material 214 from horizontalsurfaces in a directional spacer etch. The resulting structure includesspacers 216 positioned on the vertical sides of the lines 304. Thespacers 216, which have a width approximately equal to the thickness ofthe original spacer material 214 deposition, effectively narrow thewidth of the gaps 302. Preferably, the gaps 302 have a reduced width ofbetween about 500 Å and about 700 Å after the spacers 216 are formedtherein. In an exemplary embodiment, the gaps 302 have a reduced widthof about 600 Å after the spacers 216 are formed therein.

FIG. 7 illustrates a cross-section in the yz plane of the device of FIG.6 after etching a plurality of deep trenches 400 through the nitridelayer 211 and the oxide layer 210, and into the substrate 110. Thepattern of deep trenches 400 is defined according to the gaps 302between the spacers in the device array region 308. The deep trenches400 are etched using a process such as ion milling, reactive ion etching(“RIE”), or chemical etching. RIE is a directional anisotropic etchhaving both physical and chemical components. In an etching processusing a chemical etchant, such as RIE, a variety of etchants are usable,such as Cl₂. In a preferred embodiment, the deep trenches 400 are etchedto a depth of between about 3000 Å and about 5000 Å based on gaps 302,and are etched to a depth of between about 4000 Å and about 5000 Åadjacent to the wider line 306. Thus, in an example embodiment theetching technique used to define the deep trenches causes the trenchdepth to be directly proportional to the trench width.

FIG. 8 illustrates a cross-section in the yz plane of the device of FIG.7 after filling the deep trenches 400 with a spin on dielectric (“SOD”)material 408. An oxygen plasma technique is used to burn off theremaining hard mask layer 212, and a chemical mechanical polish (“CMP”)technique is used to remove the remaining spacers 216 and excess SODmaterial. The CMP technique also provides the device 100 with asubstantially planar surface 402 in the xy plane. As illustrated, thesubstantially planar surface 402 extends across the device array region308 and periphery region 310. The deep trenches 400 are separated byremaining portions of the nitride layer 211; in a preferred embodiment,the deep trenches are separated by between approximately 1600 Å andapproximately 2000 Å of nitride material. In an exemplary embodiment,the deep trenches 400 are separated by approximately 1800 Å of nitridematerial. In another exemplary embodiment, the deep trenches 400 areseparated by 2.25×F, wherein F is the minimum resolvable feature sizeobtainable using a given photolithography technique.

FIG. 9 illustrates a cross-section in the yz plane of the device of FIG.8 after patterning another hard mask layer 312 over the deep trenches400. In an exemplary embodiment, the hard mask layer 312 is patternedbased on a mask similar to that illustrated in FIG. 3, and is patternedusing photolithography and etching techniques. The patterned hard masklayer 312 defines a plurality of lines 314 over the planar surface 402,with the lines 314 effectively masking the deep trenches 400. The lines314 are separated by a plurality of gaps 318. In a preferred embodiment,the lines 314 are between about 1100 Å and about 1300 Å wide, and in anexemplary embodiment, the lines are approximately 1200 Å wide. Incertain embodiments, the lines 314 have substantially the same width asthe lines 304 formed in the masking process illustrated in FIGS. 3 and4.

FIG. 10 illustrates a cross-section in the yz plane of the device ofFIG. 9 after forming a plurality of spacer loops 316 around the lines314. In an exemplary embodiment, the spacer loops 316 are formed byfirst depositing a blanket layer of spacer material over the structureillustrated in FIG. 9. The blanket spacer material comprises an oxidematerial, such as silicon oxide having a thickness that is preferablybetween about 200 Å and about 500 Å, and more preferably between about300 Å and about 400 Å. The blanket layer of spacer material is depositedusing a suitable deposition process, such as CVD or PVD. A directionalspacer etch is then performed to remove the blanket spacer material fromhorizontal surfaces. The resulting structure is illustrated in FIG. 10.This produces a plurality of spacer loops 316 positioned on the verticalsides of the lines 314. The spacer loops 316, which have a widthapproximately equal to the thickness of the original blanket spacermaterial deposition, effectively narrow the width of the gaps 318.Preferably, the gaps 318 have a reduced width of between about 500 Å andabout 700 Å after the spacer loops 316 are formed. In an exemplaryembodiment, the gaps 318 have a reduced width of about 600 Å after thespacer loops 316 are formed.

FIG. 11 illustrates a cross-section in the yz plane of the device ofFIG. 10 after etching a plurality of shallow trenches 404 through thenitride layer 211 and the oxide layer 210, and into the substrate 110.The shallow trenches 404 are formed parallel to the deep trenches 400.In one embodiment, the shallow trenches 404 have substantially the samewidth as the deep trenches 400, but instead are etched to a reduceddepth that is preferably between about 500 Å and 2000 Å, and morepreferably between about 1000 Å and 1500 Å.

FIG. 12 illustrates a cross-section in the yz plane of the device ofFIG. 11 after filling the shallow trenches 404 with a SOD material 410.The shallow trenches are optionally filled with the same SOD material408 used to fill the deep trenches 400. A CMP technique is used toremove the remaining hard mask layer 312, spacer loops 316, and excessSOD material. In a preferred embodiment, the CMP technique is used toreduce the thickness of the nitride layer 211 to between about 300 Å andabout 500 Å. In an exemplary embodiment, the CMP technique is used toreduce the thickness of the nitride layer 211 to about 400 Å. The CMPtechnique also provides the device 100 with a substantially planarsurface 406 in the xy plane. As illustrated, the substantially planarsurface 406 extends across the device array region 308 and peripheryregion 310. FIG. 13 illustrates a top-down view in the xy plane of thedevice 100 of FIG. 12. The device 100 illustrated in FIGS. 12 and 13comprises a plurality of elongate shallow trenches 404 that areseparated from each other by elongate nitride spacers with looped ends,as defined by the remaining nitride layer 211. The nitride spacers areseparated from each other by the elongate deep trenches 400.

In a modified embodiment, the structure illustrated in FIGS. 12 and 13is obtained using a process that self-aligns in the deep trenches 400and the shallow trenches 404. As illustrated in FIG. 25, thisself-alignment is achieved by first etching the nitride layer 211 in thearray region 308. As illustrated in FIG. 26, nitride spacers 520 arethen formed around the protruding SOD material 408 structures, which nowact as mandrels. The nitride spacers 520 are then used to subsequentlypattern shallow trenches, which are etched through the oxide layer 210and into the substrate 110. The resulting structure is equivalent to thestructure illustrated in FIGS. 12 and 13, and is obtained without theuse of the hard mask layer 312 illustrated in FIG. 9.

FIG. 14 illustrates a cross-section in the yz plane of the device ofFIGS. 12 and 13 after removal of the remaining nitride layer 211 andoxide layer 210. In an exemplary embodiment, the remaining portions ofthese layers are removed using an etching process, although othertechniques are used in other embodiments. Subsequently performing a CMPtechnique results in a substantially planar surface of alternatingsilicon regions and oxide regions. The silicon regions define aplurality of elongate loops 112 that extend parallel to the x axis. Theelongate loops 112 surround shallow trenches 404, and are separated fromeach other by the deep trenches 400.

The elongate loops 112 are separated into individual transistor pillarsby etching the loops perpendicular to their length, that is, parallel tothe y axis. In certain embodiments, active devices are formed in thedevice periphery region 310 using the same masking sequence that is usedto etch the elongate loops 112 into individual transistor pillars. Insuch embodiments, active device layers are blanket deposited over thedevice illustrated in FIG. 14. The resulting structure is shown in FIG.15, which illustrates a cross-section in the xz plane of the device ofFIG. 14 after forming an oxide layer 450, a polycrystalline siliconlayer 452, and a tungsten silicide layer 454. The cross-sectionillustrated in FIG. 15 illustrates these layers formed over a siliconregion 114; however because these layers are blanket deposited, theyalso extend over the deep trenches 400 and the shallow trenches 402.Likewise, the blanket layers also extend over both the device arrayregion 308 and periphery region 310. In one embodiment, the blanketoxide layer 450 has a thickness between about 50 Å and 80 Å. In onemodified embodiment, other metallic materials are used in place oftungsten silicide to strap peripheral gates and improve lateral signalspeed. In another modified embodiment, an optional blanket siliconnitride layer (not shown) is formed over the tungsten silicide layer454. In yet another embodiment, the polycrystalline silicon layer 452comprises a conductive material, wherein the term “conductive material”includes silicon, even if undoped as deposited.

In a modified embodiment, the tungsten silicon layer 454 is omitted, andis replaced with additional thickness of the polycrystalline siliconlayer 452. This configuration advantageously removes metal from thestructure, thereby reducing the likelihood of introducing contaminationinto other structures during subsequent processing. In such embodiments,the metal is added during a subsequent silicidation process.

By patterning the blanket-deposited oxide layer 450, polycrystallinesilicon layer 452 and tungsten silicide layer 454, active devices areformed in the periphery region 310. FIG. 16 illustrates a cross-sectionin the xz plane of the device of FIG. 15 after patterning theblanket-deposited layers. In an exemplary embodiment, the layers arepatterned using photolithography and masking techniques. In theillustrated exemplary embodiment, one or more active devices 460 areformed in the periphery region 310. In such embodiments, the activedevices comprise a stack including a gate oxide 462, a polycrystallinesilicon active area 464, and a tungsten silicide strapping layer 466. Inother embodiments, the strapping layer 466 comprises other metallicmaterials, such as tungsten, titanium nitride, tantalum, and tantalumnitride. Mixtures of metals are also suitable for forming the strappinglayer 466.

Still referring to FIG. 16, the same photolithography and maskingtechnique that is used to form active devices 460 in the peripheryregion is used to pattern a series of lines 470 in the array region 308.The array lines 470 comprise the same materials as the peripheral activedevices 460, although the array lines 470 are used as a sacrificial maskto pattern the underlying elongate loops 112 in subsequent processingsteps. Additionally, the pattern of lines 470 in the array region 308has a smaller pitch as compared to the pattern of active devices 460 inthe periphery region 310. For example, in one embodiment the lines 470are spaced apart by a spacing F, wherein the active devices 460 arespaced apart by a spacing 2F, wherein F is the minimum resolvablefeature size obtainable using a given photolithography technique. Inanother embodiment, the active devices 460 have a spacing that isbetween about two times and about four times larger than the spacing forlines 470. The array lines 470, which extend parallel to the y axis, areperpendicular to the elongate loops 112, which extend parallel to the xaxis.

FIG. 17 illustrates a cross-section in the xz plane of the device ofFIG. 16 after forming silicon nitride spacers 468 around the activedevices 460 in the periphery region 310. In a preferred embodiment, thesilicon nitride spacers 468 have a thickness of between about 200 Å andabout 800 Å. In an exemplary embodiment, the silicon nitride spacers 468have a thickness of about 600 Å, and are formed by blanket depositingsilicon nitride over the device, followed by a directional etch thatremoves the deposited material from horizontal surfaces. This techniquealso results in silicon nitride spacers 468 being formed around thearray lines 470 in the array region 308. Furthermore, because thespacing between the array lines 470 is smaller than the width of twosilicon nitride spacers 468, the silicon nitride spacer material 468fills the region between the lines, thereby forming a pattern of filledgaps 472 between the lines 470. An SOD material 474, such as siliconoxide, is formed in the regions of exposed silicon. In modifiedembodiments, a material other than silicon nitride is used to form thespacers and filled gaps; other suitable materials include materials thatare selectively etched with respect to polycrystalline silicon andsilicide materials.

FIG. 18 illustrates a cross-section in the xz plane of the device ofFIG. 17 after masking the device periphery region 310 and etching gatemandrels from the device. A mask 478 is formed over the periphery region310 to protect the active devices 460 in the periphery region 310 duringsubsequent processing steps. Advantageously, the mask 478 is simple asit merely covers the periphery region 310 and opens the array 308, andtherefore does not include “critical dimension” features. After theperiphery region 310 is masked, the remaining portions of the tungstensilicide layer 454 and the polycrystalline silicon layer 452 are etchedfrom the exposed portions of the device, such as the array region 308.In an exemplary embodiment, an etchant that is selective forpolycrystalline silicon relative to oxide and nitride is used, such astetramethylammonium hydroxide (“TMAH”). Other etchants are used in otherembodiments. This results in the creation of trenches 476 between thenitride material of the filled gaps 472. In an exemplary embodiment, thesilicon is etched to the oxide layer 450, which acts as an etch stop.

FIG. 19 illustrates a cross-section in the xz plane of the device ofFIG. 18 after shrinking the remaining nitride portions of the filledgaps 472. In an exemplary embodiment, this is accomplished byisotropically etching nitride from exposed portions of the device. Asillustrated, the isotropic nitride etch advantageously creates an areaof exposed silicon/dielectric 480 as the remainder of the filled gaps472 are etched away from the remaining oxide layer 450. In an exemplaryembodiment, the remainder of the filled gaps 472 are etched to have awidth corresponding to the width of the underlying silicon elongateloops 112, illustrated in FIG. 14. In another exemplary embodiment, theremainder of the filled gaps 472 are etched to have a width of about1/2F, where F is the minimum resolvable feature size obtainable using agiven photolithography technique.

FIG. 20 illustrates a cross-section in the xz plane of the device ofFIG. 19 after etching the pattern of the trenches 476 into theunderlying structure illustrated in FIG. 14. In an exemplary embodiment,the trenches 476 are extended to an intermediate depth that is betweenthe depth of the deep trenches 400 and the shallow trenches 404,illustrated in FIG. 14. The pattern of the intermediate trenches 476 isdefined by the remaining nitride filled gaps 472. This effectively cutsthe silicon elongate loops 112, the deep trenches 400, and the shallowtrenches 404 to form a plurality of U-shaped transistor pillars. Theshallow trenches 404 form the middle gap of the U-shaped transistorpillars. In one embodiment, the U-shaped transistor pillars functionsource/drain regions for a U-shaped semiconductor structure.

FIG. 21 illustrates a cross-section in the xz plane of the device ofFIG. 20 after removing excess nitride material and forming a pluralityof sidewall spacers 482 in the intermediate trenches 476. The sidewallspacers 482 are separated from the silicon substrate 110 by a thin oxidelayer 484, such as a thermal oxide. As described herein, in an exemplaryembodiment a portion of the substrate 110 corresponding to the region ofthe elongate loops 112 is doped to include a lightly doped n⁻ region 486that is positioned underneath a heavily doped n⁺ region 488, althoughp-type doping can be employed in other embodiments. Preferably, a lowerportion of the elongate loops 112 is doped oppositely from an upperportion of the elongate loops 112. In one embodiment, the sidewallspacers 482 have a width that is greater than or equal to half of awidth of the elongate loops 112.

FIG. 22 provides a three-dimensional illustration of a portion of thepartially-formed semiconductor device of FIG. 21. As illustrated, thedevice includes a plurality of transistor pillars that form the source502 and drain 504 regions of a U-shaped transistor 500. The source 502and drain 504 regions are separated by a shallow trench 404 which runsparallel to the x axis. The channel length of the transistor is thelength extending from the source 502 to the drain 504 through theU-shaped channel region 506. The channel characteristics of the deviceare influenced by tailoring the dopant concentrations and types alongthe channel surfaces on opposite sides of the U-shaped protrusions.Neighboring U-shaped transistors 500 are separated from each other inthe y dimension by deep trenches 400, and in the x dimension by linedwith gate electrode sidewall spacers 482, which are positioned in theintermediate trenches.

FIG. 27 schematically illustrates the dimensions of a memory cell 520that is positioned in the array region 308 of a memory device. Thememory cell 520 is located at the intersection of a selected bit line522′ in a bit line array 522 and a selected word line 524′ in a wordline array 524. The periphery region 310 of the memory device optionallyincludes logic circuitry 526 that is connected to the bit line array 522and/or the word line array 524, as schematically illustrated in FIG. 27.The memory cell 520 occupies an area of the substrate 110 havingdimensions x×y, and thus size of the memory cell is generally expressedas xyF², where x and y are multiples of the minimum resolvable featuresize F obtainable using a given photolithography technique, as describedherein. The memory cell 520 typically comprises an access device (suchas a transistor) and a storage device (such as a capacitor). However,other configurations are used in other embodiments. For example, in across-point array the access device can be omitted or an access devicecan be integrated with the storage device, as in MRAM, EEPROM or PCRAM(for example, silver-doped chalcogenide glass), where the status of aswitch acts both as a switch and to store a memory state.

In the illustrated embodiment, the memory cell 520 is a DRAM cellemploying the structure illustrated in FIG. 23. The structureillustrated in FIG. 23 includes a single U-shaped transistor 500 havinga source 502 and a drain 504 separated by a shallow trench 404. Thesource 502 and drain 504 are connected by a channel region 506, which iscontiguous with the silicon substrate 110. This configurationadvantageously avoids the floating body effect that is common inconventional vertical pillar transistors. Gate electrode sidewallspacers 482 are formed perpendicular to the shallow trench 404 and looparound both sides of the U-shaped semiconductor (silicon) protrusion. Inan exemplary embodiment, a capacitor 510 or other storage device isformed over the drain 504, and an insulated bit line 512 is formed overthe source 502. As illustrated, the dimensions of the capacitor 510 andinsulated bit line 512 are large compared to the dimensions of thepitch-doubled features of the U-shaped transistor 500. In an exemplaryembodiment wherein the source 502 and drain 504 are provided with afeature size of ½F, the overlying capacitor 510 and insulated bit line512 advantageously accommodate a misalignment of up to ⅜F, wherein F isthe minimum resolvable feature size obtainable using a givenphotolithography technique. In the example embodiment that isillustrated in FIG. 23, the memory cell 520 occupies a space on thesubstrate that is preferably between about 4F² and about 8F², and ismore preferably between about 4F² and about 6.5F².

The configuration of the U-shaped transistor 500 advantageously allowsthe dimensions of the transistors that forms a part of a memory cell tobe independently scaled in the x and y dimensions, as illustrated inFIGS. 22, 23 and 27. For example, this allows a memory cell occupying anarea 6F² on the substrate to be formed with a wide variety of differentaspect ratios, including a 2.45F×2.45F square, a 3F×2F rectangle, and2F×3F rectangle. Generally, the aspect ratio of the transistorscomprising the memory device is adjustable by manipulating thedimensions of the intermediate trenches 476 and the deep trenches 400that separate the transistors.

The capacitor 510 and insulated bit line 512 are used to interface thedevice 100 with other electronic circuitry of a larger system, includingother devices which rely on memory such as computers and the like. Forexample, such computers optionally include processors, program logic,and/or other substrate configurations representing data andinstructions. The processors optionally comprise controller circuitry,processor circuitry, processors, general purpose single chip or multiplechip microprocessors, digital signal processors, embeddedmicroprocessors, microcontrollers and the like. Thus, the device 100 isable to be implemented in a wide variety of devices, products andsystems.

Referring now to FIG. 24, in certain embodiments, wafer contaminationand refresh problems are addressed by eliminating the tungsten silicidelayer 454 deposition illustrated in FIG. 15. In such embodiments, thetungsten silicide layer 454 is replaced with an extended thicknesspolycrystalline silicon layer, illustrated as layer 464 in FIG. 24.After the intermediate trenches 476 and sidewall spacers 482 are formed,as illustrated in FIG. 21, an insulating layer 490, such as a SODmaterial, is blanket deposited over the array region 308. A CMP processis then performed to expose polycrystalline silicon 464 at the tops ofthe gate stacks in the device periphery region 310. A self-alignedsilicidation process is then performed by first depositing a metal layer492. The resulting structure is illustrated in FIG. 24. Subsequently, asilicidation anneal is conducted to react the metal 492 (for example,titanium) in a self-aligned manner where it contacts the polycrystallinesilicon layer 464. Subsequently, unreacted metal 492 can be selectivelyetched, as in known in the art.

For example, in one embodiment between about 500 ∈ and about 1000 ∈ ofthe exposed polycrystalline silicon is converted to titanium silicide.Other silicide materials, such as tungsten silicide, ruthenium silicide,tantalum silicide, cobalt silicide or nickel silicide, are formed inother embodiments. This configuration advantageously allows the metaldeposition step illustrated in FIG. 15 to be eliminated, therebyreducing or eliminating metal contamination of the substrate and alsosimplifying removal of the sacrificial gate material (now just one layerof silicon) in the array 308. The embodiment of FIG. 24 takes advantageof the fact that an insulating cap layer (for example, silicon nitride)is not needed for the peripheral transistors, because the dimensions ofsuch transistors are not so tight as to require self-aligned contacts inthe region 310.

In another embodiment (not shown), a three-sided U-shaped transistor isformed. In such embodiments, the shallow trenches 404 are filled with anon-silicon oxide filler material, such as silicon nitride, at the stageof FIG. 11. Then, before forming the sidewall spacers 482 in theintermediate trenches 476, a selective etch is used to remove the fillermaterial from the shallow trenches 404. When the sidewall spacers 482are formed, semiconductor material is also formed in the shallowtrenches 404. Because the shallow trenches 404 are narrower than theintermediate trenches 476, the deposition of the sidewall spacers 482fills the shallow trenches 404. Accordingly, the subsequent spacer etchmerely recesses the gate material within the shallow trenches 404 belowthe level of the tops of the source/drain regions. This process createsa three-sided transistor structure. Advantageously, the gate materialbridges the row of U-shaped protrusions forming the sidewall gateregions on both sides and equalizing potential. Additional detailsregarding this process are provided in FIGS. 32-35 and the correspondingwritten description of U.S. patent application Ser. No. 10/933,062(filed 1 Sep. 2004; Attorney Docket MICRON.299A; Micron Docket2004-0398.00/US), the entire disclosure of which is hereby incorporatedby reference herein.

The fabrication techniques disclosed herein advantageously enable theforming of active devices in the periphery region and the patterning ofintermediate trenches in the array region with a single mask. Inembodiments wherein two are combined to define features in the peripheryand array simultaneously, a second mask is used to separate theperiphery and array regions for different subsequent processing steps.Advantageously, this second mask is not critical, and thus is easilyaligned over existing structures on the substrate. Furthermore, thefabrication techniques disclosed herein are also applicable to otherapplications. For example, such techniques are usable to form singletransistor, single capacitor DRAM cells.

In certain of the embodiments described herein, the same materials thatare used to form active devices in the periphery region 310 are alsoused as sacrificial material for subsequent masking processes in thearray region 308. Examples of such materials include the polycrystallinesilicon layer 452 and optionally, the tungsten silicide layer 454. Thisadvantageously eliminates the need to use two different critical masksto separately form features in the device periphery region 310 anddevice array region 308.

Additionally, the material used to form the gate electrode sidewallspacers 482 in the device periphery region 310 is also used as a hardmask material in the device array region 308. In one embodiment, asillustrated in FIG. 17, deposition of the silicon nitride spacers 468fill the gaps between the lines 470 in the array region 308.

SCOPE OF THE INVENTION

While the foregoing detailed description discloses several embodimentsof the present invention, it should be understood that this disclosureis illustrative only and is not limiting of the present invention. Itshould be appreciated that the specific configurations and operationsdisclosed can differ from those described above, and that the methodsdescribed herein can be used in contexts other than vertical gatedaccess transistors.

1. A memory device comprising: a substrate having an array portion and alogic portion; a first plurality of transistor devices including aplurality of semiconductor structures that are recessed in the arrayportion of the substrate, wherein a top surface of each of the pluralityof semiconductor structures is substantially co-planar with a topsurface of the substrate, and each of the plurality of semiconductorstructures include a channel portion that is contiguous with thesubstrate array portion and adjacent to a gate region; and a secondplurality of transistor devices formed over the logic portion of thesubstrate, wherein each of the second plurality of transistor devicesinclude a gate oxide layer, a conductive gate layer, and sidewall spacerstructures, wherein there is no dielectric layer between the sidewallspacer structures, and wherein the second plurality of transistordevices are formed in a layer that is above the plurality ofsemiconductor structures.
 2. The memory device of claim 1, wherein thechannel portion is between a first and second gate region.
 3. The memorydevice of claim 1, wherein the channel portion is U-shaped.
 4. Thememory device of claim 3, wherein each of the semiconductor structurescomprises a first pillar and a second pillar, and each of the firstplurality of transistor devices comprises a first source/drain regionpositioned atop the first pillar, and a second source/drain regionpositioned atop the second pillar, wherein the U-shaped channel connectsthe first and second source/drain regions.
 5. The memory device of claim1, wherein the plurality of semiconductor structures are defined by apattern of alternating deep and shallow trenches that are crossed by apattern of intermediate-depth trenches.
 6. The memory device of claim 5,wherein the alternating deep and shallow trenches are filled with anoxide material.
 7. The memory device of claim 5, wherein the alternatingdeep and shallow trenches are filled with a spin-on dielectric material.8. The memory device of claim 5, wherein the shallow trenches are atleast partially filled with a semiconductor material.
 9. The memorydevice of claim 1, wherein the conductive gate layer comprisespolycrystalline silicon
 10. The memory device of claim 1, wherein eachof the plurality of semiconductor structures occupies an area of thesubstrate array portion between about 4F² and about 8F², wherein F isthe minimum resolvable feature size formable using a photolithographictechnique that is used to define features of the semiconductorstructures.
 11. The memory device of claim 2, wherein each of theplurality of semiconductor structures occupies an area of the substratehaving a length dimension that is not equal to a width dimension. 12.The memory device of claim 2, wherein each of the plurality ofsemiconductor structures occupies an area of the substrate having alength dimension that is equal to a width dimension.
 13. The memorydevice of claim 1, wherein the plurality of semiconductor structuresform a pattern having a first pitch, and wherein the second plurality oftransistor devices form a pattern having a second pitch that is at leasttwo times the first pitch.
 14. The memory device of claim 1, wherein thesecond plurality of transistor devices are formed using aphotolithographic mask that is also simultaneously used to pattern atleast a portion of the plurality of semiconductor structures recessed inthe array portion of the substrate.
 15. The memory device of claim 1,wherein the substrate comprises single crystal silicon.
 16. The memorydevice of claim 1, wherein the second plurality of transistor devicesformed over the logic portion of the substrate are planar transistordevices.
 17. The memory device of claim 1, wherein the second pluralityof transistor devices formed over the logic portion of the substrateinclude a silicide region positioned over the conductive gate layer. 18.The memory device of claim 17, wherein the silicide region comprises amaterial selected from the group consisting of tungsten silicide andtitanium silicide.
 19. A partially-formed integrated circuit comprising:a first plurality of features comprising a first material and formedover a first portion of a substrate, wherein the first plurality offeatures are separated from each other by a first spacing; a secondplurality of features comprising a second material and formed over asecond portion of the substrate, wherein the first material is the sameas the second material; a gap fill structure positioned between andcontacting a selected two of the first plurality of features; and aplurality of sidewall spacers positioned adjacent the second pluralityof features, wherein adjacent sidewall spacers are separated from eachother by a separation region, and wherein the plurality of sidewallspacers and the gap fill structure comprise the same material.
 20. Thepartially-formed integrated circuit of claim 19, wherein the firstportion of the substrate comprises: a plurality of elongate shallowtrenches; a plurality of elongate deep trenches positioned substantiallyparallel to the plurality of elongate shallow trenches; and a pluralityof elongate silicon regions that separate the plurality of elongateshallow trenches and the plurality of elongate deep trenches.
 21. Thepartially-formed integrated circuit of claim 20, wherein the first andsecond plurality of features comprise elongate polycrystalline siliconfeatures that cross the plurality of elongate shallow trenches.
 22. Thepartially-formed integrated circuit of claim 19, wherein the first andsecond plurality of features comprise elongate gate stacks.
 23. Thepartially-formed integrated circuit of claim 19, wherein: the firstplurality of features form a pattern of sacrificial material; and thesecond plurality of features forms part of a plurality of finalstructures positioned over the second portion of the substrate.
 24. Thepartially-formed integrated circuit of claim 23, wherein the pluralityof final structures are planar transistors.
 25. The partially-formedintegrated circuit of claim 19, wherein the gap fill structure forms aportion of a hard mask pattern.
 26. The partially-formed integratedcircuit of claim 25, wherein the hard mask pattern defines trenches fora U-shaped transistor structure.